Generally a valve gear for opening and closing intake and exhaust valves at timings synchronized with rotation of a crankshaft (that is, at timings when each valve has the same rotational phase as the crankshaft) is placed in an engine. In the engine provided with such a valve gear, however, it has been known that the optimum opening/closing timings of the intake valves and the exhaust valves are, for example, changed in accordance with the running condition of the engine such as the engine speed or the engine load. Recently, it has been therefore proposed an engine including a valve gear provided with a camshaft phase varying device for varying the rotational phase of a camshaft relative to a crankshaft in order to change the opening/closing timings of intake valves and exhaust valves in accordance with the running condition of the engine.
As disclosed in JP-A-2010-31855 or JP-A-2013-7293, such a valve gear for an engine in the background art has a configuration in which a driven member, a guide member, a centrifugal weight and an urging member are installed on a camshaft individually. The driven member cannot be displaced axially relatively to the camshaft but can be displaced rotationally relatively to the camshaft. The guide member is provided on the camshaft rotatably together therewith so that the guide member can be displaced rotationally and axially relatively to the driven member. The centrifugal weight is disposed between the driven member and the guide member. The urging member urges the driven member and the guide member so as to make them close to each other. The centrifugal weight moves radially outward against the urging force of the urging member due to the effect of a centrifugal force caused by rotation of the engine. Thus, the guide member is displaced rotationally and axially relatively to the driven member so as to change the rotational phase of the camshaft relative to a crankshaft and change the opening/closing timings of valves.